1. Field of the Invention
The invention relates to a centering apparatus and a centering method each adapted to center a cylindrical workpiece set on a rotary table.
2. Discussion of Background
For example, US Patent Application Publication No. 2004/0038787 describes a machine tool that includes a centering jig attachable to a tool spindle and a runout measuring device that measures a runout due to decentering of a workpiece attached on a spindle. According to a workpiece centering method implemented by this machine tool, the runout measuring device measures the runout of the workpiece by turning the spindle. Then, the tool spindle is moved to press the centering jig against the workpiece. Based on the measured runout of the workpiece, the workpiece is pushed and centered by the centering jig.
According to the workpiece centering method described above, if the centering jig is excessively pressed against the workpiece and the axis of the workpiece passes beyond the rotation axis of the spindle, the workpiece is again decentered, resulting in another runout of the workpiece. That is, in such a case, due to the rotational force of the workpiece and the pressing force of the centering jig, the distance between the axis of the workpiece and the rotation axis of the spindle may further increase, and consequently, the workpiece may fall off the spindle.
One possible option for preventing the problem described above is to lower the speed at which the centering jig pushes the workpiece. This, however, is very time-consuming. Further, because the centering jig stops pushing the workpiece before the axis of the workpiece coincides with the rotation axis of the spindle, the runout of the workpiece is not completely eliminated, causing a certain degree of centering error.
Further, because the centering jig is attached on the tool spindle, a device that automatically switches the component attached on the tool spindle between the centering jig and a tool is required. Therefore, the switching work is very time-consuming. Further, because the centering jig is attached on the tool spindle, there is a possibility that the centering jig will come into contact with other member(s) as the tool spindle moves.